1. Field of the Invention
This invention relates to a method of deodorizing and/or deacidifying high boiling organic compounds, particularly edible oils.
2. The Prior Art
There are already numerous known methods of deodorizing and/or deacidifying organic compounds, particularly edible oils, which involve the elimination of readily volatile impurities by distillation with the aid of propellant or carrier vapors, e.g. water vapor.
Special problems arise if the material to be treated has low volatility or is virtually non-distillable and also heat-sensitive. All these conditions occur in major industrially important deodorizing processes such as for instance in the deodorization and deacidification of edible oils, the deodorization of the high-molecular weight esters of high boiling acids or alcohols such as, e.g., phthalic acid esters which are used as plasticizers, or the deodorization of high molecular weight paraffins for separating the low molecular weight fractions, and so on.
In all such cases, the heat-sensitive material from which volatile impurities are to be separated has an extremely low working pressure under the permissible pressure and temperature conditions compared with the components which are to be separated therefrom, this pressure being lower by several times the power of ten than that of the compounds which are to be eliminated. Under such working conditions, the compound can be brought to the boiling point for distillative separation only by adding a suitable amount of carrier water vapor which provides the main part of the required working pressure. Because of the thermal sensitivity the working pressures are always less than 10 Torr in the above-mentioned cases. Owing to the low condensation temperature of the carrier vapor, which is 11.degree. C., this entails a very high energy demand for the associated steam-jet vacuum-unit, amounting to a multiple of the amount of carrier steam actually required. For a given working pressure of, e.g., 6 millibar, it is directly proportional to the amount of used stripping steam.
Stripping steam demand decreases with falling working pressure but the steam consumption of the steam ejector unit increases. For reasons of economy therefore, a minimum carrier-vapor consumption at an easy to control working pressure between 1 and 10 Torr should be aimed at. This means further that, in a counterflow-multistage exchange process between descending oil and ascending stripping steam which latter gathers an increasing concentration of odour-compounds, optimum conditions are obtained if this counter-flow-exchange process can be applied at approximately constant low working pressure and rising temperatures right to the end thereof.
These conditions can only be realized in a counterflow film exchange process and for this reason there have already been various attempts to use falling-film columns for this purpose. For example, H. P. Kaufmann and K. D. Mukherjee (Fette, Seifen, Anstrichmittel--fats, soaps, paints 68,319 (1966), 70,197,370,589,801,901 (1968)) tried to obtain better results with falling-film-evaporators. However, they failed to achieve the desired success. The residual oil still had an FFA content of 0.2%, whilst the distillate was obtainable only below 90 to 95% fatty acid. Steam consumption at 200 to 300 kg/to was also very high.
One of the present inventors also undertook experiments with a view to achieving improved results by applying a counterflow falling-film process for deodorization and aiming for improved efficacy by means of an externally imposed temperature field. Whilst the application of this temperature field improved the transverse flow of the liquid particles of the falling film to the film surface, it nevertheless failed to achieve the vital breakthrough with regard to low quantities of carrier steam and less energy consumption.
Conventional modern physical processing plants used for the deacidification of palm oil operate at approximately 5 mbar and 260.degree. C. with an average consumption of steam at the rate of 30 kg/ton oil.
The present invention is aimed at achieving a considerable reduction in stripping steam consumption. A further aim of the invention resides in achieving a shorter thermal stress time for the treated high molecular weight substances and creating better heat exchange and recovery conditions between outflowing and incoming liquids. According to this invention these objects are achieved by establishing the following conditions:
1. Working pressure between 2-10 mbar, preferably 4-8 mbar. PA1 2. Working temperature below decomposition point. PA1 3. Liquid viscosity of the falling film below 0.003 Pa.multidot.s, preferably below 0.001 Pa.multidot.s. PA1 4. Reynold's number of the liquid above 100, preferably over 200. PA1 5. Pressure drop: Less than half, preferably less than 0.3 of head pressure. PA1 6. Exchanger length of trickle surface areas 6-16 m, preferably 8-12 m. PA1 7. Inside diameter of trickle channels 34-72 mm, preferably 44-54 mm. PA1 8. Liquid load per m trickle channel circumference, 0.5-4 m.sup.3 /m.h, preferably 1-2.5 m.sup.3 /m.multidot.h. PA1 9. Warm-up in falling-film column 10.degree.-20.degree. K. PA1 10. Heating medium temperature above oil-discharge temperature of max. 5.degree. K., preferably 1.degree.-2.degree. K. PA1 11. Exchange time in trickle-column of 10 to 20 s.
By applying the method according to this invention, energy consumption is significantly reduced and, for instance, in processing palm oil, a steam consumption of between 7 and 9 kg/to was achieved at comparatively short exposure or stress time for the oil.
The present invention also relates to apparatus for implementing the method. In a first embodiment of the apparatus according to this invention the counterflow-falling film evaporator consisted of a shell and tube column charged with the liquid which was to be deodorized by means of a distributor whilst the stripping steam enters the tubes from below in contraflow with the liquid. In many cases, it may be convenient and advantageous to arrange a cross-current vessel in succession with the upright shell and tube column or column with parallel honeycombchannels, to collect the treated liquid, the stripping steam inlet being then arranged inside the collecting liquid in order to eliminate any potentially occurring discolorations of the deodorized liquid.